The present invention relates to devices and methods for determining the degree of neuromuscular blockage produced by the use of muscle relaxant drugs to a patient. More specifically the present invention relates to apparatus for monitoring neuromuscular transmission.
During many surgical operations it is important to relax the patient's muscles by administering drugs such as curare, and the like. If a dose of the relaxant drug is given in the correct amount, the effects of the drug will wear off after surgery and the patient will regain his or her ability to breathe naturally, with little after-effects. If, however, an overdose is given an extended paralysis and other after effects may occur which are highly undesirable. What is needed is a way to continually measure the percentage of the completeness of the neuromuscular block produced by the muscle relaxant drug both during and after the operation. Clinical experience has shown that optimal muscle relaxation is obtained when a block is about 95% complete (5% of control). A more complete block is usually not needed and may lead to prolonged recovery or difficulty in reversing the blockage.
After surgery antagonist drugs are administered to the patient to reverse the effect of the muscle relaxants. In some cases, however, the blocking drugs originally administered to the patient may outlast the antagonists, resulting in a recurrence of weakness in the recovery room. It is therefore important to be able to monitor the degree of neuromuscular blockage during the administration of the antagonist drugs. The way in which the antagonist drugs work is to help the patient's body utilize chemicals produced within the nerve cells to reestablish control. These chemicals are subject to depletion as control is regained, producing an effect known as fading. In such a condition the initial amount of control in response to a single stimulus may be satisfactory but this control fades with repeated stimuli following immediately after the first stimulus. To check for fading it is necessary to measure the amount of neuromuscular blockage using tetanus stimulation.
Among the prior art methods and devices for monitoring neuromuscular blockage the most widely used is a portable battery operated stimulator without a meter. The anesthesiologist watches the movement of a muscle such as a finger, thumb or facial muscle after a twitch or tetanic stimulus. For research purposes, and occasionally for clinical monitoring, some prior art devices have employed strain gauges or force transducers which are taped to a digit to permit recording of the strength of the contraction. See for example U.S. Pat. Nos. 3,898,983 and 3,565,080 (Ide).
This technique requires a separate amplifier and recorder. Furthermore such devices are highly inaccurate and subject to spurious signals as the digit to which the strain gauge has been taped is moved during surgery, thus providing unreliable information. Another problem with measuring the twitch of a digit or other muscle as an indication of neuromuscular blockage is that the two are not linearly related. In some cases the muscular twitch will disappear before the EMG is fully blocked.
Another proposal is to record the EMG response from the patient. The EMG signal impulse is too fast for conventional direct writing pen recorders and the like, so that users have employed oscilloscopes and photographic recording. A recent device has been reported in which an EMG amplifier digitizes the response at 0.25 millisecond intervals and stores the digitized response in a memory, such as a magnetic tape or magnetic disc. This digitized memorized signal is then played back a few seconds later at a much slower rate into a pen recorder or the like. Aside from the time delay in obtaining the response this device also requires elaborate and expensive electronic equipment.